1. Field of Invention
The invention is directed to a quality control apparatus for a xerographic imaging system. More particularly, the invention relates to a sensorless quality control apparatus that is used to provide quality control signals to the xerographic imaging system when a quality control sensor malfunctions.
2. Description of Related Art
The basic xerographic process used in an electrostatographic printing machine generally involves an initial step of charging a photoconductive member to a substantially uniform potential. The charged surface of the photoconductive member is thereafter exposed to a light image of an original document to selectively dissipate the charge thereon in selected areas irradiated by the light image. This procedure records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document being reproduced. The latent image is then developed by bringing a developer material including toner particles adhering triboelectrically to carrier granules into contact with the latent image. The toner particles are attracted away from the carrier granules to the latent image, forming a toner image on the photoconductive member which is subsequently transferred to a copy sheet. The copy sheet having the toner image thereon is then advanced to a fusing station for permanently affixing the toner image to the copy sheet in image configuration.
In electrostatographic printing machines using a drum-type or an endless belt-type photoconductive member, the photosensitive surface thereof, can contain more than one image at one time as it moves through various processing stations. The portions of the photosensitive surface containing the projected images, so-called "image areas" are usually separated by a segment of the photosensitive surface called the "inter-document space". After charging the photosensitive surface to a suitable charge level, the inter-document space segment of the photosensitive surface is generally discharged by a suitable lamp to avoid attracting toner particles at the development stations. Various areas on the photosensitive surface, therefore, will be charged to different voltage levels. For example, there will be the high voltage level of the initial charge on the photosensitive surface, a selectively discharged image area of the photosensitive surface and a fully discharged portion of the photosensitive surface between the image areas.
The approach utilized for multicolor electrostatographic printing is substantially identical to the process described above. However, rather than forming a single latent image on the photosensitive surface in order to reproduce an original document, as in the case of black and white printing, multiple latent images corresponding to color separations are sequentially recorded on the photosensitive surface. Each single color electrostatic latent image is developed with toner of a color complementary thereto and the process is repeated for differently colored images with respective toner of complementary color. Thereafter, each single color toner image can be transferred to the copy sheet in superimposed registration with the prior toner image, creating a multi-layered toner image on the copy sheet. Finally, this multi-layered toner image is permanently affixed to the copy sheet in a conventional manner to form a finished color copy.
As described, the surface of the photoconductive member must be charged by a suitable device prior to exposing the photoconductive member to a light image. This operation is typically performed by a corona charging device. One type of a corona charging device comprises a current carrying electrode enclosed by a shield on three sides and a wire grid or control screen positioned thereover and spaced apart from the open side of the shield. Biasing potentials are applied to both the electrode and the wire grid to create electrostatic fields between the charged electrode and the shield, between the charged electrode and the wire grid, and between the charged electrode and the (grounded) photoconductive member. These fields repel electrons from the electrode and the shield resulting in an electrical charge at the surface of the photoconductive member roughly equivalent to the grid voltage. The wire grid is located between the electrode and the photoconductive member for controlling the charge strength and charge uniformity on the photoconductive member as caused by the aforementioned fields.
Control of the field strength and uniformity of the charge on the photoconductive member is very important because consistently high quality reproductions are best produced when a uniform charge having a predetermined magnitude is obtained on the photoconductive member. If the photoconductive member is not charged to a sufficient level, the electrostatic latent image obtained upon exposure will be relatively weak and the resulting deposition of development material will be correspondingly decreased. As a result, the copy produced by an undercharged photoconductor will be faded. If, however, the photoconductive member is overcharged, too much developer material will be deposited on the photoconductive member. The copy produced by an overcharged photoconductor will have a gray or dark background instead of the white background of the copy paper. In addition, areas intended to be gray will be black and tone reproduction will be poor. Moreover, if the photoconductive member is excessively overcharged, the photoconductive member can become permanently damaged.
A useful tool for measuring voltage levels on the photosensitive surface is an electrostatic voltmeter (ESV) or electrometer. The electrometer is generally rigidly secured to the reproduction machine adjacent the moving photosensitive surface and measures the voltage level of the photosensitive surface as it traverses an ESV probe. The surface voltage is a measure of the density of the charge on the photoreceptor, i.e. the photoconductive member, which is related to the quality of the print output. In order to achieve high quality printing, the surface potential on the photoreceptor at the developing zone should be within a precise range.
In a typical xerographic charging system, the amount of voltage obtained at the point of electrostatic voltage measurement of the photoconductive member, namely, at the ESV, is less than the amount of voltage applied at the wire grid of the point of charge application. In addition, the amount of voltage applied to the wire grid of the corona generator required to obtain a desired constant voltage on the photoconductive member must be increased or decreased according to various factors which affect the photoconductive member. Such factors include the rest time of the photoconductive member between printing, the voltage applied to the corona generator for the previous printing job, the copy length of the previous printing job, machine to machine variance, the age of the photoconductive member and changes in the environment.
One way of monitoring and controlling the surface potential in the development zone is to locate a voltmeter directly in the developing zone and then to alter the charging conditions until the desired surface potential is achieved in the development zone. However, the accuracy of voltmeter measurements can be affected by the developing materials (such as toner particles) such that the accuracy of the measurement of the surface potential is decreased. In addition, in color printing there can be a plurality of developing areas within the developing zone corresponding to each color to be applied to a corresponding latent image. Because it is desirable to know the surface potential on the photoreceptor at each of the color developing areas in the developing zone, it would be necessary to locate a voltmeter at each color area within the developing zone. Cost and space limitations make such an arrangement undesirable.
In a typical charge control system, the point of charge application and the point of charge measurement is different. The zone between these two devices loses the immediate benefit of charge control decisions based on measured voltage error since this zone is downstream from the charging device. This zone may be as great as a belt revolution or more due to charge averaging schemes. This problem is especially evident in aged photoreceptors because their cycle-to-cycle charging characteristics are more difficult to predict. Charge control delays can result in improper charging, poor copy quality and often leads to early photoreceptor replacement. Thus, there is a need to anticipate the behavior of a subsequent copy cycle and to compensate for predicted behavior beforehand.
Various systems have been designed and implemented for controlling processes within a printing machine. For example, U.S. Pat. No. 5,243,383 discloses a charge control system that measures first and second surface voltage potentials to determine a dark decay rate model representative of voltage decay with respect to time. The dark decay rate model is used to determine the voltage at any point on the imaging surface corresponding to a given charge voltage. This information provides a predictive model to determine the charge voltage required to produce a target surface voltage potential at a selected point on the imaging surface.
U.S. Pat. No. 5,243,383 discloses a charge control system that uses three parameters to determine a substrate charging voltage, a development station bias voltage and a laser power for discharging the substrate. The parameters are various difference and ratio voltages.
The electrostatic voltmeter 33 senses and measures the voltage on the photoreceptor belt 20 while an optical sensor device 84 measures toner mass. These measurements are used in process control loops to maintain control of the electrostatics and the development systems for the xerographic process. However, process control loops are prone to malfunction due to numerous factors such as sensor malfunction and actuator saturation. When a malfunction occurs, feedback signals from the electrostatic voltmeter 33 and the optical sensor device 84 are no longer provided and therefore the quality of the xerographic prints becomes unacceptable.
One method of avoiding this problem is to provide a backup electrostatic voltmeter and a backup optical sensor device. However, this method adds additional cost to a xerographic imaging system.
It would be advantageous if a computer-implemented sensorless controller can be used to provide the appropriate feedback signals upon malfunction of any one of the quality control sensors such as the electrostatic voltmeter and the optical sensor device.